Jill ________ biology so hard! As far as I remember, she wasn't interested in it at all. (to study)

В Present Continuous глагол to be ставится в форме настоящего времени (am, is, are) и добавляется глагол с окончанием -ing. Подлежащее Jill, следовательно, глагол to be будет в форме is.

Jill is studying biology so hard! As far as I remember, she wasn't interested in it at all.

Ответ: is studying